The present invention relates to a litz wire connector assembly and a method for making such assembly, and more particularly to a method and assembly to connect litz wire to a conductive surface while maintaining the insulative integrity of strands of litz wire to end surfaces to provide a strong mechanical and electrical connection.
At high frequency excitation, such as radio frequencies, current flows near the outside surface of a conductor. This phenomenon is known as "skin effect". This "skin effect" causes current to move away from the center of the conductor and crowd into a layer just beneath the surface. The effect is compounded in a coil, wherein the self-fields of each conductor turn induce current density changes in adjacent turns. In order to lessen the impact of skin effect, standard power wires or cables employ multiple strands of conductors whose cross section is significantly less than that of one larger conductor of the same total area. However, with increasing frequency, the impedance (resistance and inductance) of the stranded wire increases because of current crowding caused by unequal magnetic flux linkages among the individual wires.
For direct current and low frequency alternating current, the conductivity of a wire is proportional to the cross sectional area, or the square of the wire diameter. However, at high frequencies, the conductivity is directly proportional to the diameter of the wire. At high frequencies, it is desirable to maximize the conductor surface area rather than to maximize the cross sectional area of the wire.
To take advantage of these characteristics a wire may be formed by transposing individual wires within small groups of wires and then transposing the groups within the conductor. The immediate effect of this wiring method is to equalize the flux linkages of each individual strand, thus causing the current to divide evenly among the strands.
Litz wire is designed with these principles in mind. It is a conductor composed of a number of fine separately insulated strands interwoven in specific arrangements. The wire is fabricated so that all inner strands come to the outside at regular intervals, and all outer strands go to the center at equal intervals. Litz wire is basically a stranded, interwoven wire in which the conductors are insulated from each other.
It can come in numerous configurations such as a wire of sixteen (16) bundles with twenty-four (24) wire strands in each bundle. In another typical arrangement there are five (5) to nineteen (19) strands within a group. Groups are then bunched together, typically five (5) to seven (7) in a bunch. Then, bunches may be joined together, typically four (4) to seven (7) in a cable. Each of these configurations involve a helical twisting of the elements within the group. The "lay" of the wire can be tight or loose, depending on the pitch of the helical transposition.
Litz wire exhibits low losses at radio frequencies as the conducting surface area is much greater than that of an ordinary solid wire of the same diameter.
As noted above, each strand is individually insulated from other strands of the wire. Though this individual isolation is necessary for the proper functioning of the litz wire it creates a problem when the litz wire is to be terminated to a conductive surface, such as for example the surface of a printed circuit board. Particularly, in a litz wire having sixteen (16) bundles with twenty-four (24) strands in a bundle each of the individual strands are of a small diameter and there are a large plurality of these strands combined to form the litz wire. In order to develop an electrical connection between the litz wire and the conductive surface, it has in the past been common to strip or otherwise remove a portion of the insulation from the length of the strands.
Various litz wire terminations have been proposed. U.S. Pat. No. 4,412,201 to Glasauer, et al. discloses attachment of litz wire (pigtail) is accomplished by crimping it into an end of a sleeve. The litz wire lays in a groove and is captured by teeth which bite into its insulation from the inside surface of a cover thereby providing strain relief. U.S. Pat. No. 4,963,694 to Alexion, et al. is for a connector assembly for internally-cooled litz-wire cable. U.S. Pat. No. 4,475,053 to Mayer is for a brush holder for electrical machines and shows that the litz leads of brushes are resistance welded to a busbar.
Of further interest with regard to connection techniques for litz wire is a paper entitled, Suggested Soldering Techniques For Litz Conductors from New England Electric Wire Corporation having a date of May 26, 1992. The issuer of this paper notes they have been manufacturers of specialty wires for over ninety (90) years.
The suggested connection techniques cited in this paper include immersing a selected amount of litz wire into a solder pot at a high enough temperature to remove enamel (i.e. insulation) from the individually insulated strands of the wire. There is also a suggestion to use liquid flux to assist in the preparation of the litz wire. An alternative to removal by solder is the use chemical strippers which dissolve the insulation on the individual strands. A liquid and gel formulation are discussed.
The above patents and article provide various suggestions for termination of litz wire. However, the patents to Alexion, et al. and Glasauer, et al. require fairly complex mechanical construction in order to accomplish the connection. Additionally, these elements are bulky requiring a large area for the termination. The patent to Mayer, which disclosed litz leads may be resistance welded, does not clearly set forth how this is to be accomplished. It would, however, appear important to remove the insulation from the individual strands prior to welding. Also, these documents are not directly concerned with termination of litz wire to a circuit board.
The methods disclosed in, Suggested Soldering Techniques For Litz Wire Conductors include the drawbacks of requiring large amounts of solder and other toxic chemicals such as rosin flux and isopropyl alcohol, as well as chemicals used to chemically strip the insulation from the wire strands. Since these methods include chemicals which are dangerous to humans they require safeguards both for those who use the materials and for the storage of these materials. These features increase the complexity and costs of such methods.
A further drawback of the techniques now being used to terminate litz wire is the necessity to strip away insulation of the individual strands of litz wire. By this action the insulative integrity of the litz wire may be degraded. Particularly, lengths of the strands of litz wire which are stripped and, therefore, no longer insulated from each other will result in inductive linkage between the strands in an undesirable manner.
An additional drawback of the known soldering techniques is the use of a wrapping post to wrap the stripped wires around and to which they are then soldered. During this operation some strands may break due to stress placed on the individual strands, causing a loss of conduction. Further, if a post is used a significant amount of space is required.
When using solder in a solder pot to strip the insulation from the individual strands, it is necessary to maintain the solder at an extremely high temperature. When the solder is at this high temperature, chemical reactions occur where a tinning or tin replacement phenomenon occurs. In this situation, replacement of copper (i.e. the metal used for the litz strands) by the tin element of the solder takes place. If acid is used to strip the insulation material, it is then necessary to dip the stripped strands into a neutralizer and then clear water which complicates the steps required in the stripping process.
In view of the above, it is desirable to develop a simple compact assembly and a method for producing such an assembly. This desired assembly would not require the use of large amounts of toxic chemicals which can cause not only physical damage to users but also damage to the environment. The assembly and method should also result in a compact mechanically simple assembly which is economical to produce and provides a strong electrical and mechanical connection to a conductive surface. The subject invention provides these and other benefits not previously available.